The heat production of natural heterotrophic picoplankton collected in Sevastopol Bay (the Black Sea) and its long-term (from I to 105 days) enrichment batch-culture isolated from the same site was measured by isothermal microcalorimetry. Over the period of senescence of the culture, cell miniaturisation took place, with the average cell volume decreasing from 1.09 +/- 0.15 (95% CI) to 0.18 +/- 0.02 mu m(3). For the same time period, the heat fluxes decreased from 45 +/- 3 fW per cell, 56 +/- 13 fW mu m(-3) and 10 +/- 3 fW mu m(-2) to 0.5 +/- 0.2 fW per cell, 2.1 +/- 1.1 fW mu m(-3) and 0.2 +/- 0.1 fW mu m(-2), thus providing evidence of the positive dependence of the fluxes on the cell size (r(2) = 0.45, n = 68). In the natural assemblage, biovolume- and biosurface-specific heat fluxes insignificantly (r(2) = 0.19 and 0.12, respectively; n = 25) increased with decreasing average cell size from 0.75 +/- 0.12 to 0.13 +/- 0.04 mu m(3), to give indirect evidence that at least a part of the ultramicrobacterial pool are cells with high volume-specific metabolic rate. The maximum biosurface-specific metabolic rate measured for the natural bacteria proved to be close to those averaged for actively growing aquatic protozoans at 1.3 x 10(-15) mol O-2 mu m(-2) h(-1) (equivalent to 2 x 10(-13) W mu m(-2) for purely aerobic metabolism), as calculated from published data. The latter does not depend on the cell volume (r(2) < 0.001, n = 58) over the size range from 10(2) (smallest flagellates) to 10(8) mu m(3) (largest sarcodines), supplying illustrative evidence for Rubner's law. Marine bacteria (10(-1) mu m(3)) appear to fit this law and extend its scale by 2 orders of magnitude. (C) 2007 Elsevier B.V. All rights reserved.